Why doesn’t the enormous combined gravity pull the two black holes together and merge them?
Celestial mechanics most likely has them bound in a stable orbit with each other. I have to wonder though, if ultimately such an orbit would be rendered unstable due to the continued increase in the mass of the two black holes from their accumulation of debris which would result in exactly what you describe.
I wouldn't want to be anywhere nearby when that happens. I would imagine the energy release would be inimaginably stupendous.